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CHAPTER XIII ON SOME OF THE GREATER PROBIIEMS OF PHYSICAL GEOLOGY * CLARENCE E. DUTTON (b. 1841, d. 1912) The greatest problems of physical geology I esteem to be: 1st, What is the potential cause of volcanic action? Id, What is the cause of the elevation and subsidence of restricted areas of the earth's surface? 3d, What is the cause of the foldings, distortions, and fractures of the strata ? The volcanic problem is at present unsolved. Every theory or hypoth- esis thus. far offered to explain it goes to pieces at the touch of criti- cism. For elevations and subsidences we are also without any satis.factor~ explanation. But the third problem, the cause of distortions and frac- tures in the strata, looks much more hopeful, and it is my intention to propose this evening a solution of it, not a new one, let me stay, but an old one remodeled. Before proceeding to discuss it, it is proper to advert to a hypothesis which has ion;, been in favor, and which is looked upon by some authorities as affording, an explanation. This is. sometimes c.allecl the contractional hypothesis. The earth is regarded as being hot within and undergoing secular cool- i:n~ by conduction of heat through its external shell and its radiation into space. This loss of interior heat is presumed to be accompanied by a cor- responding loss of interior volume, thus. depriving, the cold. exterior shell o-i' a part. of its support. In a body so large as the earth the tangential strain set up by this. loss of interior support is demonstrably so! great that the outer shell or crust, as it is usually called, must be crushed or bucklecl by it and collapse upon the shrinking nucleus. The objection to this ex- planation is twofold: Ifs the first place, we cannot, without resorting to violent assumptions, find in this process a sufficient amount of either linear or volume contraction to account for the effects attributed to it. :[n the second place, the distortions of the strata are not of the kind which could be produced by such a process. As regards the first objection I will confine myself here to a mere reference to the very able analysis of the * This is an address read before the Philosophical Society of Washington! April 27, 1889, and published in the Bulletin of that Society, Vol. XI, pages 51-64, printed in 1892. This paper was reprinted in the Journal of the Washington Academy of Sciences, September 19,1925, Vol. 15, No. 15, and also in the Bulletin Geodesique, of the Section of Geodesy of the International Geodetic and Geo- physica.l Union, No. 9, 1926. In this paper the word " isostasy " was first used. 201
202 FIGURE OF THE EARTH problem by Rev. Os,mond Fisher. I see no satisfactory reply to his a,r~,u- -ment. As regards the second objection, which, if' possible, is more cogent still, it may be remarked that the most striking features in the facts to be explained are the long, narrow tracts occupied by belts of plicated strata and the approximate parallelism of the axes of their folds. These call for the action of some great horizontal force thrusting in one direction. Take, for example, the Appalachian system, stretching from lIaine to Georgia. Here is a great belt of parallel synclinals and a,nticlinals with a persistent trencl, and no rational inquirer can doubt that, they have been puckered up by some vast force acting horizontally in a northwest and southeast direction. Doubtless it is the most wonderful example of systematic pli- cation in the world. :E3ut there are many others which indicate the opera- tion of the same forces with the same broad characteristics. The par- ticular characteristic with which we are here concerned is that in each of these folded belts, the horizontal force has acted wholly or almost wholly in one direction. But the forces which would arise from a col- lapsing crust would act in every direction equally. There would be no determinate direction. In short, the process could not form long, narrow belts of parallel folds. As I have no time to discuss the hypothesis further I dismiss it with the remark that it is quantitatively insufficient and qualitatively inapplicable. It is an explanation which explains nothings, hich we want to explain. In proposing another view of the problem we may first turn our atten- tion to those obvious and universally conceded forces which determine the figure of' the earth. That figure we know to be one which a liquid or v I ~ 1 viscous body of large size will take when subject only to the forces arts- in~, from rotation arouncl~ an axis and to the mutual gravitation of its own parts. This form is an oblate spheroid. The spherical form, however, is only approximate. We find large por- tions of its surface protruding into continents and islands, while others are sunken to form oceanic basins. How did these inequalities arise? If' the I'orm of the earth is nearly spheroidal why is it not exactly so? It has always been supposed that this nearly spheroidal form implies that the earth, if not liquid, is certainly not rigid enough to maintain ~,. . · . near stem Term ~.~ninst the Torces of its own gravitation. Even if' the ~ 1 I By ~ ~11 I_ 1 1 V ~ 44~ ~ ~ ~ e ~ U V^ & ~ ~ ~ ^ ~ ~ \~ ~ ~ vie ~ ~ tt) ~ . . r -I ~ ~ ~ ~ ~1 ~ to I 1 · _ 1~ earth were a mass of unbroken steel no great, ctepa,rture from ants s~l,~e could be maintainecl for a n~on~ent. It would straightway collapse and flow into a spheroidal form. But if' gravitation compels it to take a nearly spheroidal shape why should it stop short of making it perfectly so ? Perhaps it will be said that while the rigidity of rocks may be insuffi cient to permit a great deformation of' the normal spheroid it may be suffieicnt to permit a small one. Before discussing this point it will be
SOME PROBLEMS OF PHYSICAL GEOLOGY 203 necessary to intro~lu.ce a co~si.deration which has seldom been. touched upon by geographers or geologists. If the earth were composed of homogeneous matter its normal figure of equilibrium without strain would be a true spheroid of revolution; but if hetero~eneo;us, if some parts. were denser or lighter than others, its normal figure would no longer be spheroidal Where the lighter mat- ter was accumulated there would lie a tendency to bulge, and where the denser matter existed there would be a tendency to flatten or depress the surface. For this condition of equilibrium of figure, to which gravitation tends to reduce a planetary body, irrespective of whether it be homo- geneous or not, I propose the name isostasy. I would have preferred the word isobar::, but it is preoccupied. MTe may also use the corresponding adjective, isostatic. An isostatic. earth, composed of homogeneous matter and without rotation, would be truly spherical. If slowly rotating, it would be a spheroid of two axes. If rotating rapidly within. a certain limit, it might be a spheroid o:l three axes. But if the earth be not homogeneous. if some portions near the sur- face be lighter than others then the isos.ta.tic figure is no longer a sphere or spheroid of revolution, but a deformed figure bulged where the matter is light and depressed where it is heavy. The question which I propose is: How nearly does the earth's figure approach to isostasy? Mathematical statics alone will not enable us to answer this question with a sufficient degree of approximation. It does, indeed, enable us to ]x certain limits to the clepa.rture from isostasy which cannot be exceeded. This very problem has been treated with great skill by Prof. George Darwin. But this problem may be approached from another direction with more satisfactory results. Geology furnishes us with certain facts which enable us to draw a much narrower conclusion. There are several categories of fact to which we may turn. One of the most remarkable is the general fact that where Breast bodies of strata are deposited they progressively settle down or sink seemingly by reason of their t,ross mechanical. weight, just as a railway embankment across a bog sinless into it.. The attention ~ . . . Of the ea.rller Appalachian geologists was called, as soon as they had a.c- quired a fair knowledge of their field, to the surprising fact that the Paleozoic strata. in that wonderful belt, though tens of thousands of feet in thickness, were all deposited in comparatively shallow water. The Paleozoic beds of the Appalachian region have a thickness ranting from 10,000 to over 30,000 feet, yet they abound in proofs that when they revere deposited their surfaces were the bottom o:l a shallow sea. whose depth could not probably have exceeded a. few hundred feet. No conclu- sion is left us but that sinking went on part passe with the accumulation
204 of the strata. When the geology of t FI G URE OF THE EAR TH the Pacific coast was sufficiently dis closed, the same fact Gonfront.ed us there. As investigation. went on. the same fact presented itself over the western mountain region of the United States. One of the most striking cases is the Plateau Country. This great region, nearly 100,000 square miles in area, lying in the adjacent parts of Colorado, fJtah, New Mexico, and Arizona, discloses from 8,000 to 12,000 feet of mesozoic and Cenozoic strata. Here the proof is a.bun- dant that the surface of the strata was throughout that vast stretch of time never snore than a few feet from sea level. Again and again it emerged from the water a little way, only to be submerged. At, many horizons grew forests. which are now represented by those abundant and beautiful fossil woods which of late have become celebrated. In the cre- taceous we kind many seams and seamless of coal or carbonaceous shale; but they are inclucled. between sandstones which. are cross-bedded and ripple-marked, or between shales and limestones which abound in the remains of' marine mo;llusca. Here the evidence seems conclusive that the whole subsidence went on at about the sane rate as the surface was built up by deposition. In short, it may be laid down as a, general rule that where great bodies of sediment have been deposited over extensive areas their deposition hats been accompanied. by a subsidence of the whole mass. The second class ol' facts is even more instructive, and stands in a reciprocal relation to those just mentioned. Wherever broa.cl ~:noulltain plat:t'orms occur and have been subjected to great erosion the loss of' alti- tucle by degradati.o~-~ is made good by a' rise o:t' the platform. Ill. the west- ern portion o-l' the United States there occur mountain ranges situated upon broad and lofty platforms from 20 to 60 miles wide and from 50 to 200 miles in length. Some of these platforms contain several n~oun- tain ridges. All of' them have been enormously eroded, and if the matter removed from them could be replaced it would suffice to build them to heights of eight or ten miles; yet it is incredible that these mountains were ever much loftier than now, and may never have been so lofty. The flanks of these platforms, with the upturned edges of the strata reposing against them or with gigantic faults measuring their immense uplifts, plainly declare to us. that they have been slowly pushed upwards as fast as they were degraded by secular erosion. It seems little doubtful that these subsidences of accu~nu.lation (1e,- posits and these progressive upward movements of' eroded mountain platforms are, in the main, results of gravitation restoring the isosta,sy which has been disturbed by denudation on the one hand and by sed.i- mentation~ on the other. The magnitudes of' the masses which thus show the isostatic tendency are in some cases no greater than a single moun
SOME PROBLEMS OF PHYSICAL GEOLOGY 205 fain platform, less than 100 miles in length, from 20 to 40 miles wide and from 2,500 to 3,500 feet mean altitude above the surrounding lowlands. Prom this we may directly infer that in those regions the effective rigidity of the earth is insufficient to uphold a mass so great as one of those plat- I'orms if that mass constituted a real deformation of isosta.sy; and if an equal mass were to be suddenly removed the earth would flow upward I'rom below to fill the hiatus; hence we must look to considerably smaller masses to find a de-f'ect of isosta.sy. It is extremely probable that small or narrow ridges are not isosta.tic with. respect to the country round about. them. Some volcanic mountains may be expected to be non-isostatic, especially isolated volcanic piles. Thus the geologic changes which have taken place may be regarded as experiments conducted by Sat.ure herself on a Vast scale, and from her experiments w ~ may by suitable working hypotheses draw provisional conclusions, both as to the degree in which the earth approximates to isost.asy and also as to the mean effective rigidity of large portions of the subterranean mass. The approach to isostasy is thereby inferred to be very near, while the mean rigidity of the subterranean masses is. also inferred to be far less than that of ordinary surface rocks,, and. even ap- proaching more nearly the rigidity of lead than to that of copper. Pure physics alone would not have enabled us to reach. such a conclusion, for the equations employ constants of unknown value. But geologic inquiry may, and I believe does, furnish us with narrow limits within which. those values must be talked Thus the two sciences must work cooperatively and supplement each. other. There is, however, one other branch of physical inquiry which bears directly on the fore~oin,~, questions. This is the investi~a,tion oft ter- restria,1 gravitation by means of' the pendulum. I regret that I have never had time or opportunity to acquaint myself thoroughly with the results thus far reached by this branch of investigation, and can only speak from general knowledge. Pendulum observations are far too few for the wants of t,eo~raphic or `,eolot,ic science. So far as they ,t,o~ they are highly su~,- ~,estive in the present connection. The pendulum., as. a rule, does, not show any appreciable variation of gravity, such as would be expected if the mean density of all flee outer parts of' the earth were uniform. It ind;- cates rather that the elevated regions and continents, are composed of' lighter n~at.ter and the depressed regions a.ncl ocean basins of denser ~at- ter. The exceptions are of' a character which prove the general rule, and occur where we should fool; for them. The results obtained by the In.d;a survey upon the R;malayan mass were re~arcled by Archdeacon Pratt as indicating, that the plateau was composed of' lighter matter than the low- lands to the southwards A similar result has been obtained in the threat ~. ~. ~,
206 FIGURE OF THE EARTH bulge which forms the western half of the Unitecl States. In other words, the pendulum indicates that those elevated regions are nearly if not quite isostatic. ()n the other hancl, the observations of Mend.enh.all on Fujiva~na., in Japan, indicated a slight excess of mass, and a similar result would seem to follow from Mr. Preston's world in the Hawaiian Islands. From the rrature of the process by which volcanoes are built these results are to be expected. It would also seem natural to expect that the plun~b-line would give some incli.cations upon this subject; but experience has shown. that most of the observed deflections of the plumb-line are inexplicable. They oc- cur where we would least expect them upon broad and. level plains, Here there is nothing to indicate any cause of cleflection. The: are found on the tundras of Siberia and the monotonous expanse of British North America, where the surface of the earth is but feebly diversified. Ire mou:rltain regions they are often conflicting and unintelligible, but along the sea coast. the indications are more systematic. On both the Atlantic and Pacific shores the deflection of the plummet is almost invariably towards the ocean, and is often of considerable amount; but it is along the shore that the isostatic theory would lead us to look for just this de- flection, for it is along the margins of the continents that great bodies of sediment accumulate; and so long as the earth possesses any note- worthy degree of rigidity, enabling it to sustain in part the resulting (reformation of isostas.y, so long must we expect to kind these sediments constituting- an excess of masts whose attraction will make itself felt upon the plummet. The theory of isostasy thus briefly sketched out is essentially the theory of Babbage and Herschel, propounded nearly a century ago. It is, however, presented in a modified form, in a new dress, and in greater detail. We may now proceed to deduce some important consequences. A little reflection must satisfy us that the secular erosion of the land and the deposit of sediment along the shore lines constitute a continuous disturbance of isostasy. The land is ever impoverished of material is continuously unloaded; the littoral is as continuously loaded up. The resultant forces of gravitation tend to elevate the eroded land and to de- press the littoral to their respective i.sostatic levels. Whether these forces shall become kinetic and produce actual movement or flow will depend., - first, upon their intensity; secon.d, upon the ri~idity of the earth by which such movement its resisted. Let us consider, then, the intensity of the forces. The littoral belts upon which sediments are throw n down are co- extensive in length with shores. Their widths are no doubt variable, but
SOME PROBLEMS OF PHYSICAL GEOLOGY 207 must often reach a hundred miles. or more with considerable thickness, and are not wholly unimportant at much greater distances. The thick- ness of the deposits may vary much, but may be proportional to the time of accumulation, and here time is measured by the geologic standarcl. The gross weight of such masses of sediment must be vast indeed.. If there is any viscous yielding, at all the problem becomes essentially that of the flowing solid, which is in a large measure governed by hydrostatic laws. The intensity of the :force must have a maximum value propor- tional to the thickness which lies above the isostatic level and also pro- portional to its specific gravity. The area covered by the deposit enters as a quantity factor, but not as an intensity factor. The greater the area, the greater is the total potential energy of movement without any neees- sary increase of the intensity of the force. This intensity, being propor- tional to the thickness of the sediments, may become almost indefinitely great or it may be small. Indeecl, it may, and in fact does, become nega- tive when we apply the same statical theory to the movement or stress. of the denuded land areas But whether these forces are sufficient to produce actual flow is equally dependent upon else rigidity, or, as we may here term it, the viscosity of the masses involved. We have already seen reason to infer that the mean viscosity is not great, being far less than that of the surface rocks alone. Beyond this rather vague statement I perceive no way of assigning, a value to the resistance to be overcome. It remains to inquire what is the resulting direction of motion. The general answer is, towards the direction of least resistance. The specific answer, which must express the direction of least resistance, will, of course, turn upon the configuration of the deposition on the one ha.ncl, and of denudation on the other, and also upon the manner in which the rigidity or viscosity varies from place to place. Taking, then, the case of a land area undergoing denudation, its detritus carried to the sea and deposited in a heavy littoral belt, we may re~a.rd the weight of each ele- menta.ry part of the deposited mass as a statical force acting upon a vis- cous support below. Assuming that we could find a deferential expres- sion appliea.ble to each and every element of the mass and a. c.orrespond.illg one for the resistance offered by the viscosity, the integration for the en- tire mass might give us a series of equipotential surfaces within the mass. The resultant force at any point of any equipotential surface would be normal to' that surface. A similar construction may be applied to the a.d- joining denuded area., in which the defect of isosta.sy may be treated as so much mass with a negative algebraic sign. The resultants normal to the equipotentia.l surfaces would in this case, also have the negative sign. The effective force tending to produce movement would be the a.rith 14
208 FI CURE OF TlIE EAR TH metical sum of the normals or of a single resultant compounded of the two normals. From this construction we may derive a force which tends to push the loaded sea. bottoms inward upon the unloaded land hori- zontally. This gives us a force of the precise kind that is wanted to explain. the origin of systematic plications. Long reflection and considerable analysis have satisfied me that it is sufficient both in intensity and in amount unless. we assume for the mean Viscosity of the superficial and subter- ranean masses involved in the movement a much greater value than I am disposed to concede. The result is ~ true viscous flow of the loa.decl littoral inward upon the unloaded continent. There may be in this proposition some degree of violence to a certain mental prejudice against the idea that the rock-ribbed earth, to which all our notions of stability and immovableness are attached, earl be made to flow. It may assist our efforts if we reflect upon the motion of the great ice sheet which covers Greenland. Ilere the masses involved are no greater than some masses of sediment. The specific gravity of ice is only about one-third that of the rock masses. The forces called into play to carry the glacier along horizontally do not seem to diner greatly in intensity or amount from the described forces, and the rigidity of the ice itself may not exceed the mean rigidity of the rock masses beneath the littoral. We may now proceed to inquire how this theory adjusts itself to the actual facts. And, firstly, where do systematic pliGa.tions occur ? 1) It is a remarkable fact that they occur among sedimentary beds of great and variable thickness., which were rather rapidly accumulated. They seldom, and., so far as I now recall, never occur among strata which are of small thickness, slowly accumulated with uniformity over large areas; and the theory requires that they should occur in the heavy de- posits or along their margins., and should have their greatest develop- ment there, for the forces called into play must be proportional to the masses involved. 2) They occur in their systematic form along the ancient shore lines. This is but another way o:t statical the preceding proposition. It hats its uses, however, for in so far as the continents have preserved approximately their old shore lines since the ages in which the plications were formed there is a conspicuous. parallelism of the axes of plica.tion to the neigh- boring coast. This is true of the Pacific. coast of the United States. As re~a.rds the Appalachian plications, we have the remarkable fact that in Paleozoic time the ocean lay to' the west of those vast bodies of folded strata instead of to the east of them, as now. We must fool: to a Paleozoic Atlantis for the origin of a Breast portion of those sediments. The flow of the earth was from west northwest to east southeast.
SOME PROBLEMS OF PHYSICAL GEOLOGY 209 3 ~ The parallelism of the folds and their occurrence in long, narrow belts formed by horizontal forces acting in one direction become a conse- quence so obvious as to need no' comment. It is in strong contrast with the contractional theory, which gives a force without any determinate clirection. 4) Another important fact is that these systematic flexures were mainly formed at the times the sediments were deposited. This is a fact of' geologic observation. The contractional hypothesis gives no determi- nate time for the formation of these flexures. It holds up to us a. process continuous through all geological time, proceeding at a rate which dimin- ishes but slowly as the ages roll by. These plica.tions, according to the isostatic theory, are the results of the disturbance of isos.tasy, and follow immediately upon that disturbance or after it has reached a sufficient amount, and cease with it. These folds, however, hate been subject since their first formation to great erosion, which is. also a disturbance of i.so~s.tasy, and thus the original plication may have been increased or modi- fied thereby. The theory may also be applied in a most satisfactory manner to the explanation of subordinate: features associated with plication. S) One of the features of plica.tion which has attracted great atten- tion and occasioned great. perplexity to geologists is the so-called fan- structure. This is very striking in the Alps, and has its counterpart in the inclined folds of the Appalachians of Pennsylvania, where the north- western branches of the anticlines. are steeper than the southeastern hra.nches. If we assume that as the rocks lie deeper in the earth they are softened somewhat by the increas.in~, heat, it follows that in the flow of the mass the movement would be easier and more rapid below than above. Thus a horizontal force arising from this differential movement acts upon the inverted arches of the synclines and carries their lower vertices for- ward in the direction of motion. Thus the general theory here proposed gives an explanation of the origin of plications. It gives us a. force acting in the direction required in the manner required, at the times. and places required, and one which has the intensity and amount required and no more. The contractional theory gives us a force having neither direction nor determinate mode of action, nor definite epoch of action. It gives us a force acting with a far greater intensity than we require, but with far less quantity. To pro- vide a place -for its action. it must have recourse to an arbitrary postulate a.ssumin`, for no independent reason the existence of areas of weakness in a supposed crust which would have no raisorr d'etre except that they are necessary for the salvation of the hypothesis.
210 FI CURE OF THE EAR TH Before closing this discussion it will be necessary to advert to another one of the great problems of physical geology, viz., the cause of general eleva.tio~ns and subsidences. I do so, not with the idea of throwing light upon it, but to guard against a misapprehension which would otherwise be sure to occur. Geologic history discloses the fact that some great areas of the earth's surface which were in former ages below sea level are now thousands of feet above it. It also gives us reason to believe that other areas now sub- merged were in other ages terra 1?rrna. Our western mountain region al; the beginning, of Cenozoic time was at sea level. It is now, on an average, 6,000 feet above it. The great Himalayan. plateau contains. early Cenozoic beds full of marine fossils which now lie at altitudes of 14,000 feet or more. The whole North American Continent has, since the close of the Paleozoic, gained in altitude. Now, it is sufficiently obvious that the theory of isosta.sy offers no explanation of these permanent changes of level. On. the contrary, the very idea of isostasy means the conservation of profiles against lowering by denudation on the land and by deposition on the sea bottom, provided no other cause intervenes to change those levels. If, then, that. theory be true, we must look for some independent principle of causation which can gradually and permanently change the profiles of the land and sea bottom. And I hold this cause to' be an independent one. It has been much the habit for geologists to attempt to explain the progressive elevation of plateaus and mountain platforms, and also the foldings of the strata by one and the same process. I hold the two processes to be distinct and as having no necessary relation to each other. There are plicated regions which are little or not at all ele- `-ated, and there are elevated regions. which are not plicated. Plication may go on with little or no elevation in one geologic age and the same . ~ . , ~ . ,. , . ~ ~ ... . .. .. . . region may be elevated without much additional pl~cat~o~n In a subse- quent age. This is in a large measure true of the Sierra. Nevada plat- :lorm, which was intensely plicat.ed during the Paleozoic and early meso- zoic, but which received its present altitude in the late Cenozoic. Whatever may have been the cause of these great regional uplifts it in no manner affects the law of isostasy. What the real nature of the up- lifting force may be is, to my mind, an entire mystery; but I think we may discern at least one of its attributes, and that is a gradual expansion, or a diminution of the density, of the subterranean magmas. If the isostatic force is. operative at all, this expansion is a rigorous conse- quence; for whenever a rise of the land has taken place one of two things hats happened; the region affected has either gained an accession of mass or a mere increase of volume without increase of mass. We l~now of no cause which could either add to the mass or diminish the density, yet . . . . . .
SOME PROBLEMS OF PHYSICAL GEOLOGY 211 one of the two must surely have happened. But the difference of the two alternatives in respect to' consequences is immense. If the increase of' volume of an elevated area be due to an accession of matter, the plateau must be hoisted against its own rigidity and also against the statical weight of its entire mass lying above the isostat,ic level. But if' the in- crease of' volume be due to a decrease of density there is no resistance to be overcome in order to raise the surface. Hence I infer that the cause which eles-a,tes the land involves an expansion of' the underlying ma~- mas, and the cause which depresses' it is a shrinkage ol' the magmas. Tl~e nature of the process is, at present, a complete mystery.
